Carrying value as of the balance sheet date of short-term borrowings using unsecured obligations issued by banks, corporations and other borrowers to investors. The maturities of these money market securities generally do not exceed 270 days.

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Face (par) amount of debt instrument at time of issuance.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

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Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility.

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The carrying value as of the balance sheet date of the current portion of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement.

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The cash inflow during the period from additional borrowings in aggregate debt. Includes proceeds from short-term and long-term debt.

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Weighted average interest rate of short-term debt outstanding calculated at point in time.

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Debt - Narrative (Details)	3 Months Ended
Debt - Narrative (Details)	Mar. 31, 2020 CAD ($)	Mar. 31, 2020 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Debt Instrument [Line Items]
Commercial paper borrowings	$ 28,000,000		$ 20,000,000	$ 397,000,000
Commercial Paper
Debt Instrument [Line Items]
Current borrowing capacity			$ 1,000,000,000.0
Commercial paper, Weighted-average interest rate	2.55%		2.55%	2.03%
U.S $1.0 Billion Tranche
Debt Instrument [Line Items]
Current borrowing capacity			$ 1,000,000,000.0
U.S. $300 Million Tranche
Debt Instrument [Line Items]
Current borrowing capacity			300,000,000
Revolving credit facility, Drawn	$ 142,000,000		$ 100,000,000
Revolving Credit Facility
Debt Instrument [Line Items]
Debt instrument, Interest rate	1.875%		1.875%
Current borrowing capacity			$ 1,300,000,000
2.050% 10-year Notes
Debt Instrument [Line Items]
Debt instrument, Face amount			$ 500,000,000
Debt instrument, Interest rate	2.05%		2.05%
Debt instrument, Term	10 years	10 years
Debt instrument, Maturity date	Mar. 05, 2030	Mar. 05, 2030
Net proceeds from issuance of debt	$ 662,000,000	$ 495,000,000
3.050% 30-year Notes
Debt Instrument [Line Items]
Debt instrument, Face amount	$ 300,000,000
Debt instrument, Interest rate	3.05%		3.05%
Debt instrument, Term	30 years	30 years
Debt instrument, Maturity date	Mar. 09, 2050	Mar. 09, 2050
Net proceeds from issuance of debt	$ 296,000,000